...by separating out CPPFLAGS and CXXFLAGS.

This doesn't seem needed, and the empty include was breaking the
build.

CPPFLAGS should contain -I and -D flags.

This also has the effect of fixing the build of this module.

...including one syntax error and an underspecified cast.

some potential fixes for srcdir, need someone with deps to tell me what doesn't actually work here now

Mike removed mod_voipcodecs from tree in commit
bcd9f49f.

We'd long since replaced mod_voipcodecs with mod_spandsp.

Things seem to go wrong when building mod_ldap with make -j.

This allows building mod_soundtouch in clang as clang doesn't support
gcc's -fcheck-new.